Apoplastic pH is a chemical switch for extracellular H2O2 signaling in abscisic acid‐mediated inhibition of cotyledon greening

Summary The apoplastic pH (pH Apo ) in plants is susceptible to environmental stimuli. However, the biological implications of pH Apo variation have remained largely unknown. The universal stress phytohormone abscisic acid (ABA) as well as the major environmental stimuli drought and salinity were selected as representative cases to investigate how changes in pH Apo relate to plant behaviors in Arabidopsis . Variations in pH Apo negatively regulated the cotyledon greening inhibition to the universal stress hormone ABA or environmental stimuli through the action of extracellular hydrogen peroxide (eH 2 O 2 ). Further studies revealed that an increase in pH Apo diminishes the chemical reactivity of eH 2 O 2 , effectively functioning as an ‘off’ switch for its action in oxidizing thiols of plasma membrane proteins. Consequently, this suppresses the eH 2 O 2 ‐mediated cotyledon greening inhibition to environmental stimuli and ABA, alongside inhibiting the eH 2 O 2 ‐mediated intracellular Ca 2+ signaling. Conversely, a decrease in pH Apo serves as an ‘on’ switch for the action of eH 2 O 2 . In summary, the pH Apo is a crucial messenger and chemical switch for eH 2 O 2 in signal transduction, notwithstanding the apparent simplicity of the underlying mechanism. Our findings provide a novel fundamental biological insight into the significance of pH.